Fourier transform infrared (FTIR) spectrometers are utilized in the analysis of chemical compounds. In these instruments, a beam of infrared radiation having a band of infrared wavelengths is passed into an interferometer, typically a Michelson interferometer, and is modulated before being passed through the compound to be analyzed and then to a detector. The interferometer modulates the radiation received by it to provide an output beam in which a narrow range of infrared wavelengths is typically reduced or enhanced in intensity, with the affected range of wavelengths changing periodically over time. The time correlated output data from the detector is analyzed by Fourier transformation to derive information on the characteristics of the sample through which the beam was passed.
In the typical Michelson interferometer used in such FTIR spectrometers, the input beam is received by a beam splitter which partially passes the beam through to a moving mirror and partially reflects the beam to a fixed mirror, or vice versa, and the reflected beams are recombined at the beam splitter to yield the output beam. The relative position of the moving mirror with respect to the beam splitter and fixed mirror will determine which wavelengths constructively and destructively interfere when the beams from the two mirrors are recombined at the beam splitter. The movement of the moving mirror toward and away from the beam splitter results in the scanning of the constructively and destructively interfering wave lengths across a desired band of infrared wave lengths. Examples of such Michelson interferometer systems in FTIR instruments are shown in U.S. Pat. Nos. 4,799,001 and 4,847,878.
It is critical in the design of FTIR instruments that the surfaces of the fixed mirror and the moving mirror be accurately held orthogonal to each other. Mirror position accuracy is crucial because deviations in the mirror alignment produce small errors in the time domain interferogram which may translate into large errors in the frequency domain spectrum. In a typical interferometer, mirror deviations larger than one wavelength of the received radiation beam are considered significant and can seriously degrade the quality of the instrument.
Static alignment of the mirrors of the interferometer is typically accomplished by means of differential screws at the back of the mirror which are manually adjusted to align the mirror to a desired position as perfectly as possible. This is a time consuming procedure requiring skill and experience, and adds to manufacturing expense and to field service costs because realignment in the field is often required.
Efforts have been made to eliminate the need to manually align the interferometer mirrors. Automatic static alignment at least relieves the user from performing time consuming realignments. For example, stepper motors have been used to carry out automatically the manual alignment procedure described above. Such devices typically use a digital computer which aids in the alignment of the static mirror at periodic service intervals. A disadvantage of this approach is the slow speed, large size, high cost, and high precision bearings required for the alignment mechanism. To attempt to adjust the moving or fixed mirror dynamically to compensate for the tilting of the moving mirror as it moves on its bearing requires more speed that can be readily obtained with a mechanism using lead screws and stepper motors. Another approach has been to use piezoelectric positioners to align dynamically the tilt of the mirrors. Such positioners are also typically large and expensive, and require high voltage (e.g., 1000 volts) drive levels. The power supplies required for such high voltages also create undesirable operating hazards as well as being relatively expensive.